Process for polishing silicon wafers

ABSTRACT

A process for the chemical-mechanical polishing of silicon wafers is by rotational movement of the silicon surface which is to be polished on a polishing plate which is covered with polishing cloth, with a continuous supply of an alkaline polishing agent which contains abrasives, at least 2 μm of material being removed from the polished silicon surface during the polishing. Immediately after the polishing has finished, and while maintaining the rotational movement, instead of the polishing agent at least two different stopping agents are supplied in succession, each removing less than 0.5 μm of material from the polished silicon surface.

BACKGROUND OF THE INVENTION

[0001] 1. Field of the Invention

[0002] The present invention relates to a polishing process for semiconductor wafers made from silicon, which are used in particular in industry for the fabrication of microelectronic components.

[0003] 2. The Prior Art

[0004] Semiconductor wafers made from silicon, which are used as substrates for the fabrication of modern microelectronic components have to fulfill a wide range of properties. These properties are often specified within relatively narrow limits. A number of these quality parameters are only determined in the final processing step of the wafers, generally involving polishing followed by cleaning. Properties of this type include, for example, the planarity of the silicon wafers, their surface roughness and the extent of surface defects, such as scratches, spots and localized light scatterers.

[0005] The polishing is generally carried out as a chemical-mechanical process. Here the silicon surface which is to be polished, while an alkaline polishing agent which contains abrasives is being supplied, is moved in rotation over a polishing plate covered with polishing cloth. The alkalinity of the polishing agent leads to chemical dissolution of the amphoteric silicon, while the abrasives, for example silicon dioxide particles (SiO₂), have an additional mechanical action.

[0006] In the prior art, various types of polishing processes have been disclosed. It is possible to subject the silicon wafers to a single-side polishing process, in which the wafer is held on the back surface by a support device by means of vacuum, adhesion or wax and is polished on the front surface. The rear surface is not affected by the polishing and remains in the original state, which is determined, for example, by a previous grinding or etching process. It is also known to carry out the single-side polishing as a two-stage, three-stage or four-stage process. Here the individual stages are carried out on different polishing machines which are covered with polishing cloths of different hardness, and this process is in widespread use in practice. In each case the final step is carried out with a relatively soft cloth, in order to achieve the desired, low roughness values.

[0007] In addition, processes for the simultaneous polishing of front surface and back surface of silicon wafers using the double-side polishing process are known. This process is now being used to an increasing extent in particular for the industrial manufacture of semiconductor wafers with diameters of 200 mm and 300 mm. In this process, the semiconductor wafers are moved, in carriers which have suitably dimensioned cutouts, over a path which is predetermined by the machine and process parameters. These wafers are moved between two rotating polishing plates, which are covered with polishing cloth, in the presence of an alkaline polishing agent, and in this way are polished with a high plane-parallelism being produced. Therefore, double-side polishing, unlike single-side polishing, provides silicon wafers with a polished front surface and a polished back surface. For cost reasons, double-side polishing is not carried out in a plurality of stages, but rather is generally followed by a brief single-side polishing of the front surface, in order to produce a surface of low roughness.

[0008] Both types of polishing process result in a highly reactive, hydrophobic, i.e. water-repelling, silicon surface. The thin native oxide film which is normally present on the silicon surface has been removed. The silicon surface is exposed to continued attack from alkaline polishing agent and atmospheric oxygen, being present at the end of the silicon-removing polishing. Therefore, methods which protect the reactive wafer surface immediately after the polishing have been developed. By way of example, an improvement is brought about if the supply of alkaline polishing agent, toward the end of the polishing, is replaced by a supply of ultrapure water, while rotation continues. However, incomplete rinsing away of alkaline polishing agent residues may lead to spots being formed on the wafer surface.

[0009] EP 684 634 A2 has described a single-side polishing process for semiconductor wafers which is distinguished by two different polishing agents of different grain size being successively supplied on one polishing machine. In a preferred embodiment using two different polishing agents based on SiO₂ (“silica sol”), first of all a mixture of polishing agent 1 with alkali additives (pH greater than 11), and then polishing agent 2 with alkali additives (pH greater than 11), and then an acidic aqueous stopping agent based on polyalcohol, hydrogen peroxide (H₂O₂) and acid (pH less than 4) are supplied. There are a number of drawbacks associated with this process. Firstly, the roughness of the polished silicon surface which is produced does not allow direct further processing. For example deposition of an epitaxial coating is not allowed or the fabrication of components is not allowed, without following the process with an expensive smoothing touch polishing step. Secondly, the addition of alkali to the polishing agent 2 is associated with the formation of spots on the silicon surface. Thirdly the considerable drop in pH is caused by the addition of the acidic stopping agent, particularly when using large polishing machines which enable 12 or more silicon wafers to be polished simultaneously. As a result of coagulation of the silica sol particles, there is therefore the destruction of the sol. This leads to the formation of hard SiO₂ crystallites, leading to scratches being formed on the polished surfaces. The reason is the slow neutralization and acidification of the relatively high quantities of alkali which are bound in the polishing cloth, with the result that the pH range of approximately 4 to 6.5. This is critical for the destruction of the sol and therefore causes the formation of crystallites, and is only passed through slowly.

[0010] An improved process for stopping the polishing process is described in the German Patent Application which has the reference number DE 199 38 340.5, which involves adding a stopping liquid containing a polyhydric alcohol in order to stop the double-side polishing process. In a preferred embodiment, the stopping liquid additionally contains small quantities of butanol and surfactant. The significantly easier reduction in the pH, with simultaneous complete coverage of the freshly polished wafer surface, means that the wafers polished in this way are substantially free of scratches and spots. On account of their roughness, these wafers are suitable for the deposition of an epitaxial coating of, for example, silicon without having to use a smoothing touch polishing step. However, there remains a need to improve the roughness values after polishing, in order to increase the yields of epitaxially coated silicon wafers and of microelectronic components and therefore to further reduce the production costs thereof.

SUMMARY OF THE INVENTION

[0011] Therefore, it is an object of the present invention to provide a process for polishing silicon wafers which have lower roughness and defect rates on the polished surfaces than wafers which have been polished in accordance with the prior art.

[0012] The above object is relieved by the present invention which provides a process for the chemical-mechanical polishing of silicon wafers by rotational movement of a silicon surface which is to be polished on a polishing plate which is covered with polishing cloth, with a continuous supply of an alkaline polishing agent which contains abrasives, at least 2 μm of material being removed from the polished silicon surface during the polishing, and immediately after the polishing has finished, and while maintaining the rotational movement, instead of supplying the polishing agent, supplying at least two different stopping agents in succession, each removing less than 0.5 μm of material from the polished silicon surface.

[0013] It is a significant feature of the present invention that the material-removing polishing step is successively followed by two different stopping steps whose properties are adapted to one another. The first stopping step has the object of smoothing the wafer surface; the second stopping step is responsible for cleaning and preserving. In this context, a clear distinction is drawn between a polishing step, in which at least 0.5 μm of material, for example 2 to 25 μm, is removed from each polished side of the silicon wafer, and a stopping step. In a stopping step, less than 0.5 μm of material, for example 0 to 0.3 μm, is removed from each polished side of the silicon wafer. The fact that a sequence of steps of this nature allows silicon wafers with improved surfaces to be produced was unexpected, was surprising and was impossible to foresee.

[0014] The starting product for the process is silicon wafers with rounded edges which are produced by sawing a silicon single crystal and have been subjected to one or more of the process steps of lapping, grinding, etching and polishing. The end product of the process is silicon wafers with a polished front surface and an unpolished back surface or a polished front surface and a polished back surface, at least one polished side having a low roughness and a low defect rate.

[0015] The process according to the invention can in principle be used to produce wafer-like bodies which consist of a material which can be treated using a chemical-mechanical polishing process. The use of single-crystal silicon wafers is particularly preferred and forms the subject of the following description.

[0016] The silicon wafers which are produced by sawing a silicon single crystal and rounding the edges may be subjected to one or more abrasive process steps before the polishing process according to the invention is carried out. The inventive process has the object of improving the wafer geometry and of removing flawed surface layers and contaminations. Suitable processes are, for example, lapping, grinding and etching. Wafers with polished surfaces can also be subjected to the process according to the invention. This is done for example in order to rework wafers which have already been polished but do not comply with specifications, so as to transform them into a state in which they do comply with the relevant specifications.

[0017] The process according to the invention can be used for any polishing technique which operates in accordance with the chemical-mechanical polishing principle. In the case of the polishing of silicon wafers, this means, to achieve the planned removal of material by working with a polishing agent which is generally supplied continuously. In addition to the abrasive solid particles which bring about mechanical abrasion, the polishing agent also contains an alkaline component, which allows the silicon surface, which is no longer covered by a native oxide layer, to be attacked chemically.

[0018] Both single-side polishing and double-side polishing are suitable for execution of the process according to the invention. For cost and quality reasons, it has proven expedient for at least 12 silicon wafers to be polished simultaneously, and this is therefore preferred. On account of its higher technological potential, in particular with regard to the planarity and topology of the silicon wafers, double-side polishing is particularly preferred for the production of wafers to supply modern component lines within the scope of the invention. The statements made below in connection with the polishing process according to the invention are therefore directed only to the double-side polishing process.

[0019] A double-side polishing machine which is suitable for carrying out the process substantially comprises a lower polishing plate, which can rotate freely in the horizontal plane, and an upper polishing plate, which can rotate freely in the horizontal plane. Both of these plates are covered, preferably by adhesive bonding, with in each case one polishing cloth. This machine, with a continuous supply of the alkaline polishing agent, allows abrasive polishing of silicon wafers on both sides. The silicon wafers are in this case held, during polishing, on a geometric path which is determined by machine and process parameters, preferably on a cycloidal path, by carriers which have suitably dimensioned cutouts for receiving the silicon wafers.

[0020] The carriers are in contact with the polishing machine, for example by means of a pin gearing or an involute toothing, via a rotating inner pin ring or toothed ring and an outer pin ring or toothed ring. It generally rotates in the opposite direction, and as a result are set in rotary motion between the two polishing plates. It is particularly preferable to simultaneously use from four to six planar carriers made from stainless chromium steel which are covered with in each case at least three silicon wafers. The edges of these wafers are protected by polymer linings in the cutouts. By way of example, within the scope of the invention it is possible for 30 silicon wafers with a diameter of 200 mm (distributed over 5 carriers each comprising 6 silicon wafers) or 15 silicon wafers with a diameter of 300 mm (distributed over 5 carriers each comprising 3 silicon wafers) to be polished simultaneously on a commercially available polishing installation of suitable size.

[0021] The preferred diameter of the silicon wafers to be polished is between 150 and 300 mm. In present-day specifications from the component manufacturers, this relates to the diameters 150 mm, 200 mm and 300 mm. However, intermediate sizes and slightly larger or smaller diameters are also possible. If the diameters were considerably smaller, the quantity to be polished at the same time would generally be excessively high to allow speedy loading and unloading. For larger diameters, for example 400 mm or 450 mm, it is possible in principle to use the process according to the invention. But the preferred configuration with at least twelve wafers to be polished at the same time leads to polishing machine dimensions which have not yet been feasible to produce.

[0022] Polishing is particularly preferably carried out using a commercially available polyurethane polishing cloth with a hardness of from 60 to 90 (Shore A), which may contain incorporated polyester fibers. The polishing process takes place with a continuous supply of a polishing agent which contains abrasives and the pH of which has been set to the desired level by the addition of alkali. Suitable polishing agents are aqueous suspensions or colloids of a large number of abrasive inorganic substances, for example silicon dioxide, silicon nitride, silicon carbide, aluminum oxide, titanium dioxide, titanium nitride, zirconium dioxide or cerium dioxide, to which an alkaline substance is added. These alkaline substances which are added can possibly be sodium carbonate (Na₂CO₃), potassium carbonate (K₂CO₃), further alkaline carbonate compounds, sodium hydroxide (NaOH), potassium hydroxide (KOH), ammonium hydroxide (NH₄OH), tetramethylammonium hydroxide (TMAH) and/or further alkaline hydroxide compounds, in proportions of from 0.01 to 10% by weight, and also, if appropriate, other additives in small proportions. The percent by weight of the alkaline substance is based upon the total weight of the aqueous polishing agent.

[0023] According to the invention, an aqueous polishing agent which contains silicon dioxide (SiO₂) with a grain size of between 5 and 50 nm and a solids content of from 1 to 10% by weight and a pH of between 9 and 12 is preferred. The weight percent of the solids content is based upon the total weight of the aqueous polishing agent. The SiO₂ content is particularly preferably derived from precipitated silicic acid of chemical formula Si(OH)₄, and the pH is between 10.5 and 12. Polishing is preferably carried out under a polishing pressure of from 0.1 to 0.5 bar. The silicon removal rate is preferably between 0.1 and 1.5 μm/min, and particularly preferably between 0.4 and 0.9 μm/min. The amount of silicon removed is preferably between 2 and 25 μm per polished wafer surface.

[0024] After the polishing step has ended, the chemically wholly reactive, hydrophobic wafer surface has to be passivated. Within the context of the invention, this is achieved by, immediately after polishing has finished, and without opening the polishing machine, successively supplying at least two different stopping agents, the rotational conditions being maintained but with the pressure being reduced significantly to between 0.005 and 0.1 bar, preferably between 0.01 and 0.05 bar. Between the supply of the at least two different stopping agents, it is possible for ultrapure water without further additives to be supplied for a brief time. In this case, the stopping agents fulfill different functions: the first stopping agent is responsible for smoothing the silicon surface by removing streaks and lowering the surface roughness without removing significant quantities of material. The second stopping agent is responsible for cleaning the wafer surface and preserving it, for example by producing an oxide or by application of a film of liquid. To avoid strong changes in pH, which lead to the formation of SiO₂ crystallites and therefore to the occurrence of surface scratches on the silicon wafers, it is preferable for the first stopping agent to have a lower pH than the polishing agent and for the second stopping agent to have a lower pH than the first stopping agent. The differences in pH, in a particularly preferred embodiment, are no greater than 3 pH units. A pH of between 9 and 10.5 is particularly preferable for the first stopping agent, and a pH of between 7.5 and 9 is particularly preferable for the second stopping agent, if the polishing agent has a pH of between 10.5 and 12.

[0025] The role of the first stopping agent is fulfilled particularly well by a weakly alkaline aqueous suspension which contains from 0.1 to 5% by weight of SiO₂ particles of a grain size of between 5 and 50 nm. The percent by weight of SiO₂ particles is based upon the total weight of the first stopping agent suspension. The SiO₂ particles were produced by pyrolysis of Si(OH)₄ (“pyrogenic silica”), to which a polyhydric alcohol has been added in a proportion of from 0.01 to 10% by volume. The percent by volume is based upon the total volume of the suspension of the first stopping agent. The polyhydric alcohol prevents, by condensation reaction with the hydrophobic silicon surface, which has Si—H terminal groups, a significant chemical attack from residual polishing agent and the weakly alkaline stopping agent from occurring. Also the spherical form of the pyrogenic SiO₂ particles leads to smoothing of the silicon surface without significant quantities of material being removed.

[0026] The polyhydric alcohol is preferably selected from the list of compounds and compound classes consisting of glycerol (1,2,3-propanetriol), monomeric glycols, oligomeric glycols, polyglycols and polyalcohols. Examples of suitable monomeric glycols are ethylene glycol (1,2-ethanediol), propylene glycols (1,2-propanediol and 1,3-propanediol) and butylene glycols (1,3-butanediol and 1,4-butanediol). Examples of suitable oligomeric glycols are diethylene glycol, triethylene glycol, tetraethylene glycol and dipropylene glycol. Examples of polyglycols are polyethylene glycol, polypropylene glycol and mixed polyethers. Examples of polyalcohols are polyvinyl alcohols and polyether polyols. Said compounds are commercially available, often in different chain lengths in the case of polymers. The first stopping agent may additionally contain small quantities of short-chain, monohydric alcohols, such as i-propanol and n-butanol, and of surfactants. The term surfactant is understood as meaning a surface-active agent.

[0027] The role of the second stopping agent is fulfilled in particular by an aqueous solution which contains a film-forming agent or a plurality of film-forming agents. The concentration used is dependent on the nature of the film-forming agent and lying between 10-4 and 50% by volume. A concentration range between 0.01% and 10% by volume is generally preferred. The percent by volume of the film forming agent is based upon the total volume of the second stopping agent aqueous solution.

[0028] Substantially two demands are imposed on the film: (1) It must protect the surface of the silicon wafer from residues of alkaline liquid which are present on the polishing cloth and from atmospheric oxygen. (2) It must be possible for the film to be completely removed by cleaning. Within these parameters, the chemical composition of the film-forming agent may in principle be selected as desired. Within the context of the invention, it is preferable to use an agent or a plurality of agents selected from the list of compound classes comprising polyhydric alcohols and surfactants. In this context, it should be noted that some alcohols, particularly oligomeric and polymeric polyhydric alcohols, have surfactant properties.

[0029] Examples of suitable polyhydric alcohols are the alcohols described above in connection with the description of the first stopping agent. It is possible, although not imperative, for the first and second stopping agents to contain the same polyhydric alcohol. An example of a surfactant is a preparation based on alkylbenzenesulfonic acid and amine ethoxylate, which is commercially available from ICB under the trademark SILAPUR®. The second stopping agent may in addition contain short-chain, monohydric alcohols, such as i-propanol and n-butanol, in concentrations of from 0.01 to 2% by volume.

[0030] The first and second stopping agents are in each case supplied for a period of preferably 0.1 to 10 min, particularly preferably for 0.5 to 5 min. The silicon wafers, which are completely covered with the film after the second stopping agent has been supplied and the polishing machine has been opened, are removed from the polishing machine and are subjected to cleaning and drying in accordance with the prior art. The cleaning may be carried out either as a batch process with simultaneous cleaning of a multiplicity of wafers in baths or by spraying processes or as an individual-wafer process. Within the context of the invention, it is preferable to use bath cleaning in which all the wafers from one polishing operation are cleaned simultaneously, for example using the sequence comprising aqueous hydrofluoric acid—ultrapure water—aqueous TMAH/H₂O₂ solution—ultrapure water, megasound assistance in the TMAH/H₂O₂ bath being advantageous in order to improve the removal of particles. For spot-free drying, there are commercially available units which operate, for example, according to the centrifugal drying, hot-water, Marangoni or HF/ozone principle, all of which are equally preferred.

[0031] The double-side-polished wafers obtained in this way are dry, hydrophilic and free of spots, scratches and further flaws which are visible under focused light. They are obtained in high yields and have very low surface roughnesses, as demonstrated, for example, by AFM (“atomic force microscope”) or Chapman measurements. Consequently, they have advantages over silicon wafers which have been polished in accordance with the prior art and can without problems be passed either to a reduced touch polishing stage followed by further processing or to an epitaxial coating or component fabrication stage immediately after the polishing step according to the invention. By way of example, quality inspection for light spot or haze defects after the application of the epitaxial coating using the process according to the invention leads to yields which are approximately 10% higher than when using processes of the prior art.

BRIEF DESCRIPTION OF THE DRAWINGS

[0032] Other objects and features of the present intention will become apparent from the following detailed description considered in connection with the accompanying drawings. It should be understood, however, that the drawings are designed for the purpose of illustration only and not as a definition of the limits of the invention.

[0033] In the drawing, wherein similar reference characters denote similar elements throughout the several views:

[0034]FIG. 1 shows the process sequence of a single-side polishing process with sequential supply of two polishing agents and one stopping agent in accordance with the prior art;

[0035]FIG. 2 shows the process sequence of a double-side polishing process with sequential supply of one polishing agent and one stopping agent in accordance with the prior art, as described in the Comparative Example; and

[0036]FIG. 3 shows a preferred process sequence according to the invention for a double-side polishing process with a polishing agent and two stopping agents being supplied sequentially, as described in the Example.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS

[0037] Weakly boron-doped monocrystalline (100)-silicon wafers (resistance range 10-30 Ω.cm), which had been edge-rounded, ground and acid-etched, with a diameter of 300 mm and a thickness of 805 μm were provided for the Comparative Example and the Example. Moreover, there were five carriers made from stainless chromium steel with a thickness of 770 μm, which each had three circular cutouts which were arranged at regular intervals along a circular path, were lined with polyvinylidene difluoride and had an internal diameter of 301 mm. This allowed the simultaneous polishing of 15 silicon wafers on a double-side polishing machine of type AC2000 produced by Peter Wolters. For this purpose, the carriers had circumferential teeth which fitted onto the inner and outer involute teeth of the polishing machine. The upper and lower polishing plates of the polishing machine were covered with in each case one polishing cloth of type SUBA500 produced by Rodel, which was composed of a layer of polyurethane foam reinforced with polyester fibers facing the silicon wafers which are to be polished, a moisture barrier layer and a pressure-adhesive layer.

COMPARATIVE EXAMPLE

[0038] The double-side polishing step was carried out using an alkaline polishing agent which comprised an aqueous suspension of precipitated silica (Sio₂ particle size 10-20 nm; solids content 3% by weight; NaOH-stabilized) and, after the addition of alkali (2% by weight of K₂CO₃ and 0.03% by weight of KOH), had a pH of 11.2. The polishing took place under a pressure of 0.15 bar and with the upper and lower polishing plates at a temperature of in each case 40° C., leading to a removal rate of 0.65 μm/min. After the thickness of the polished wafers reached 775 μm, the supply of the polishing agent was ended and was replaced, for a period of 3 min, by the supply of a stopping agent which comprised an aqueous solution of 1% by volume of glycerol, 1% by volume of n-butanol and 0.07% by volume of a commercially available surfactant known under the trademark SILAPUR® (produced by ICB). The lower polishing plate, the upper polishing plate and the carriers continued to be moved and the pressure was reduced to 0.03 bar. A sample of the stopping agent had a pH of 8.0. The polished silicon wafers were removed from the polishing machine and were cleaned in a batch-cleaning installation using the bath sequence comprising aqueous hydrofluoric acid—ultrapure water—TMAH/H₂O₂/megasound—ultrapure water, and were dried in a commercially available hot-water dryer. The silicon wafers produced in this way were substantially free of scratches, haze spots and localized light scatterers.

EXAMPLE (INVENTION)

[0039] The procedure was as described in the Comparative Example, except that between the polishing step and the stopping step, with the rotary conditions maintained, a further stopping step followed by a brief feed of ultrapure water, both at a pressure of 0.03 bar, was added. The corresponding stopping agent 1 comprised an aqueous suspension of pyrogenic silica (SiO₂ particle size 30-40 nm; solids content 1.5% by weight; NH₄0H-stabilized), to which 0.3% by volume of triethylene glycol was admixed and which had a pH of 9.7. Therefore, to stop the polishing process, the liquids listed below were supplied sequentially: (1) stopping agent 1 (SiO₂/triethylene glycol in ultrapure water; 3 min); (2) ultrapure water (2 min); (3) stopping agent 2 (glycerol/butanol/surfactant in ultrapure water; 2 min). The silicon wafers produced in this way were likewise substantially free of scratches, haze spots and localized light scatterers.

[0040] Determination of the Roughness

[0041] The surface roughness of the silicon wafers which had been polished in accordance with the Comparative Example and the Example was determined using an optical measuring instrument using the phase difference of a linearly polarized, split laser beam, with a part-beam being reflected from the wafer surface (“Chapman process”). When using different filters, the following mean values for the roughness were obtained for the RMS (root mean square) values for these long-wave roughnesses. The roughness of the front surfaces and of the back surfaces of the simultaneously polished wafers were identical within the margin of error. For the silicon wafers produced in accordance with the invention Example, they demonstrate significantly lower roughnesses in all filter regions compared to the silicon wafers produced in accordance with the Comparative Example: Roughnesses Filter Comparative example Example 250 μm 0.59 nm 0.32 nm  80 μm 0.43 nm 0.16 nm  30 μm 0.29 nm 0.08 nm  10 μm 0.12 nm 0.03 nm

[0042] Accordingly, while a few embodiments of the present invention have been shown and described, it is to be understood that many changes and modifications may be made thereunto without departing from the spirit and scope of the invention as defined in the appended claims. 

What is claimed is:
 1. A process for the chemical-mechanical polishing of silicon wafers comprising rotational movement of a silicon surface which is to be polished on a polishing plate which is covered with polishing cloth; continuously supplying an alkaline polishing agent which contains abrasives, and removing at least 2 μm of material from the polished silicon surface during polishing; immediately after the polishing has finished, and while maintaining the rotational movement, instead of supplying the polishing agent supplying at least two different stopping agents in succession; each stopping agent removing less than 0.5 μm of material from the polished silicon surface.
 2. The process as claimed in claim 1, comprising polishing 12 silicon wafers simultaneously on one polishing machine.
 3. The process as claimed in claim 1, comprising simultaneously polishing of a front surface of the silicon wafers and a back surface of the silicon wafers, with from 2 to 25 μm of material being removed from each surface; and guiding the silicon wafers between two oppositely rotating polishing plates covered with polishing cloth by carriers with cutouts which are suitably dimensioned to receive silicon wafers.
 4. The process as claimed in claim 1, wherein the first stopping agent smooths the polished silicon surface, and the second stopping agent cleans and preserves the polished silicon surface.
 5. The process as claimed in claim 1, wherein the pH of the first stopping agent is lower than that of the polishing agent and the pH of the second stopping agent is lower than that of the first stopping agent.
 6. The process as claimed in claim 1, wherein the polishing agent substantially comprises a colloidal mixture of 1 to 10% by weight of SiO₂, based upon the total weight of the polishing agent, in water with the addition of alkali and has a pH of between 10.5 and 12.0; the first stopping agent substantially comprises a colloidal mixture of 0.1 to 5% by weight of SiO₂, based upon the total weight of the first stopping agent, in water with the addition of a polyhydric alcohol and has a pH of between 9.0 and 10.5; and the second stopping agent substantially comprises a solution of a polyhydric alcohol in water and has a pH of between 7.5 and 9.0.
 7. The process as claimed in claim 6, wherein the polyhydric alcohol which is used in the first stopping agent and the polyhydric alcohol which is used in the second stopping agent each comprises a compound selected from the group consisting of glycerol, monomeric glycols, oligomeric glycols, polyglycols and polyalcohols and is used in proportions of from 0.01% to 10% by volume, based upon the total volume of the stopping agent.
 8. The process as claimed in claim 6, wherein the first stopping agent and the second stopping agent additionally contain small quantities selected from the group consisting of monohydric alcohols and surfactants.
 9. The process as claimed in claim 6, wherein the alkali added to the polishing agent comprises at least one compound selected from the group consisting of Na₂CO₃, K₂CO₃, NaOH, KOH, NH₄OH and tetramethylammonium hydroxide, in proportions of from 0.01% to 10% by weight, with the percent by weight based upon the total weight of the polishing agent.
 10. The process as claimed in claim 6, wherein the polishing agent contains, as the SiO₂ component, precipitated silica with a grain diameter of between 5 and 50 nm; and the first stopping agent contains, as the SiO₂ component, pyrogenic silica with a grain diameter of between 5 and 50 nm.
 11. The process as claimed in claim 6, wherein ultrapure water is supplied between the supply of first stopping agent and the supply of second stopping agent. 